Microwave duplexer



Oct. 9, 1962 Filed Nov. 4, 1959 l. REINGOLD ETAL MICROWAVE DUPLEXER FERRITE FILTER-LIMITER 2 Sheets-Sheet l FERRITE FILTER-LIMITER MY 3-DECIBEL 3 COUPLED- STRIP-LINE 7- DIRECTIONAL COUPLER RECEIVER DC MAGNETIC FIELD OF STRENGTH H APPLIED TO FILTER-LIMITERS TO TUNE THEM TO OPERATING SIGNAL STRENGTH POWER IN REc'R ARM SAFE LIMIT FOR DETECTOR POWER 1N ANTENNA ARM [8 ESSENTIALLY EQUAL TO POWER OUT OF TRANS- MITTER.

POWER OUT OF TRANSMITTER ATTENUATION IN REc FREQ INVENTORS, JOHN L. CARTER IRVING R INGOLD.

A TTUR/VE K.

Oct. 9, 1962 I. REINGOLD ETAL MICROWAVE DUPLEXER 2 Sheets-Sheet 2 Filed Nov. 4, 1959 D mwP-C 55213:. 20th INVENTORS JOHN L. CARTER IRV/NG REl/VGOLD.

A TTORNE Y.

Patented Oct. 9, 1962 3,953,079 MHCRGWAVE DUPLEXER Irving Reingold, Deal Park, and John L. Carter, Ashury Park, N.J., assignors to the United States of America as represented by the Secretary of the Army Filed Nov. 4, 1959, Ser. No. 850,974 1 Claim. (Cl. 333-9) (Granted under Title 35, U.S. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereon.

The invention relates in general to microwave signal transmission systems, and particularly to duplexing equipment for directionally controlling signal transmission in microwave radar and other two-way radio communication systems employing a common antenna for transmission and reception.

It is more specifically directed to such duplexing equipment of the balanced type for conditioning the associated radar or similar radio system alternately for efiicient signal transmission and reception at the same microwave frequencies.

A general object of the invention is to produce the duplexing operations in such systems efficiently and economically with equipment that is relatively small, compact and simple in construction; requires no movable devices, gas-filled tubes or other thermionic devices for performing the necessary signal control actions and, therefore, has a relatively long, useful life; and has desirable bandpass, tuning and timing characteristics.

A primary object is to enable the use of the same antenna for signal radiation and reception at the same microwave frequencies in a radar or like two-way radio communication system while preventing damage to the sensitive detector elements in the receiver thereof by the high-power outgoing signal energy during signal transmitting intervals without reducing excessively the relatively low-power signal input to the receiver during signal receiving intervals.

A related but more specific object is to introduce signal control actions into the branching circuits connecting the signal transmitter and receiver to the common antenna in such a radar or like communication system, such as to enable system operation alternately for signal transmission and reception with minimum adverse efiects on the quality of signal transmission and minimum damage to sensitive system components by high-power signal energy.

The non-linear phenomena exhibited by ferrites or other ferrimagnetic material as a function of incident wave power level are utilized in the design of lumped-element resonant networks or resonators employing such material for producing the necessary control actions in the duplexer of the invention to attain the above objects. The active element of each resonator comprises a small amount of a ferrite or other ferrimagnetic material, such as single crystal garnet, in the form of a small sphere or bead, connected in, or closely electrically coupled to, a wire conductor common to the transmission paths for the highpower outgoing signal energy and the relatively low-power incoming signal energy of the same microwave frequency in the associated radar or other radio communication system, the resonator being tuned to this signal frequency by the application of a direct current magnetic field of suitable strength transversely to the sphere or bead. With the elements of this resonator properly proportioned, it will operate in response to the incident high-power energy received over the associated Wire conductor from the transmitter of a radar or similar system during signal transmitting intervals to provide a power limiting action on that energy such as to prevent transmission of any appreciable amount of it through the resonator to the receiver of the system, and instead to cause reflection of substantially all of this energy through associated circuits to the common antenna of the system for radiation thereby; and in signal receiving intervals when the relatively low-power incoming signal energy from the associated antenna is applied thereto over the associated wire conductor, the resonator will operate as a narrow-band filter tuned to the signal frequency to transmit essentially all of that energy therethrough with low loss to the associated receiver of the system for detection therein. The filter formed by the resonator also protects the receiver against spuriouslygenerated wave components outside the narrow bandwidth of the filter.

A balanced duplexer in accordance with one embodiment of the invention employs two such resonators tuned to the common operating signal microwave frequency of the transmitter and receiver of the associated radar or like system by an applied transverse direct current magnetic field of suitable strength, coupled through two directional couplers of a conventional coupled-strip-line type in balanced bridge relation with the transmitter, receiver and common transmitting and receiving antenna of the associated radar or like communication system and a suitable non-refiective dummy load. In this duplexer, the two ferrite resonators serve to provide the above-described power-limiting and reflecting action on the outgoing signal energy and filtering action on the incoming signal energy, respectively, and the directional couplers serve to provide the necessary combination of the incoming and the direct and reflected outgoing signal energy components in phase and amplitude in transmission between the resonators and the associated apparatus elements of the communication system, to provide optimum duplexing operation in that system.

A feature of the duplexer in accordance with the invention is that it is tunable over a wide range of microwave signal frequencies by adjustment of the strength of the direct current magnetic field applied to the ferrimagnetic material in the resonators.

These and the other features and objects of the invention will be better understood from the following complete description thereof when it is read in conjunction with the several figures of the accompanying drawings, in which:

FIG. 1 shows a block schematic diagram of a balanced duplexer embodying the invention applied to a microwave radar or similar two-way radio communication system;

FIG. 2 is a combination perspective view of a practical embodiment of the balanced duplexer of the invention shown in FIG. 1, partially broken away to show structural details more clearly, and a schematic diagram of the connections between the filter-limiter network or resonator and directional coupler portions thereof and the transmission apparatus of the associated radar or radio communication system; and

FIGS. 3 and 4 show curves used to illustrate the operation of the duplexer of the invention in controlling the operation of the associated radar or like communication system in the signal transmitting and receiving conditions, respectively.

Referring to FIG. 1, the balanced duplexer of the invention shown therein comprises the substantially identical ferrite filter-limiter networks 1 and 2 connected between two four-terminal, 3-decibel directional couplers 3 and 4 of a conventional coupled-strip-line type. The input and output of the network 1 are respectively connected to one of the four terminals of the directional coupler 3 and one of the four terminals of the directional coupler 4; and the input and output of the network 2 are respectively connected to a second terminal of the directional coupler 3 and a second terminal of the directional coupler 4. The transmitter 5 and common transmitting and receiving antenna 6 of a radar or like radio communication system are respectively connected to the third and fourth terminals of the directional coupler 3; and the receiver 7 of this same system and a non-reflective dummy load of such value as to effectively balance the impedance of the antenna 6 are respectively connected to the third and fourth terminals of the directional coupler 4, so as to provide a balanced bridge connection between these four system elements in which the two ferrite filter-limiter networks 1 and 2 are respectively included in oppositely-disposed bridge arms. As indicated by the arrows so labeled, the ferrite filter-limiter networks 1 and 2 are tuned to the common operating microwave frequency of the transmitter and receiver of the associated system by the application transversely to these networks from an associated external source (not shown) of a constant direct current magnetic field of a suitable strength H.

As shown in the practical embodiment of the balanced duplexer of the invention in FIG. 2, the ferrite filterlimiter network 1 may comprise a lumped-element resonator including as its active element a small sphere or bead 1a made from single crystal garnet and closely electrically coupled to a wire conductor 1b; and the ferrite filter-limiter network 2 may comprise a substantially identical lumped-element resonator including as its active element a similar small sphere or bead 2a made from single crystal garnet and closely electrically coupled to the wire conductor 2b, the garnet sphere or beads 1a and 2a being subjected transversely to a direct current magnetic field of suitable strength to tune the resonators 1 and 2 to the same microwave frequency which is the common operating frequency of the signal transmitter 5 and signal receiver 7 of the associated microwave radar or radio communication system.

Each of the directional couplers 3 and 4 used in the balanced duplexer of the invention may be of the general coupled-strip-line type described in the article entitled, Coupled-Strip-Transmission-Line Filters and Directional Couplers by E. M. T. Jones and J. T. Bolljahn, in the I.R.E. Transactions on Microwave Theory and Techniques, vol. MIT-4 No. 2, April 1956, pp. 75-81, and more completely in chapters 3 and 4 of the final report on a research project on strip transmission lines and components sponsored by the Signal Corps Engineering Laboratories, Fort Monmouth, NJ. (Contract DA63-039 SC-63232), published by the Stanford Research Institute of Menlo Park, California in February 1957. As described in these publications, the basic directional coupler of this type may comprise a pair of strip transmission lines which are closely electrically coupled to each other over a portion of their lengths. This device, when its four terminal pairs are terminated in the proper constant resistance, operates as a directional coupler with infinite directivity and constant impedance theoretically available at all frequencies and for all degrees of coupling. The amplitude of the coupled wave varies approximately sinusoidally with frequency, with maximum coupling occurring when the length of the coupling region i an odd multiple of a quarter wavelength. If a signal is fed into one terminal of the device, the coupled signal energy emerges from the two opposite terminals; no signal energy emerges from the adjacent terminal.

In the practical embodiment of the duplexer of the invention illustrated in FIG. 2, the directional couplers 3 and 4 are mounted on a sheet 9 of dielectric material, such as Teflon, the opposite faces of which form ground planes. The dimensions of this sheet 9 would be selected such as to reduce the fringing effects in the couplers mounted thereon to a minimum. As shown, the directional coupler 4 has two longitudinally-extending thin, fiat copper strips 10 and 11 embedded in the upper surface (top ground plane) of the dielectric sheet 9, including central straight portions extending longitudinally in closely parallel relation to each other for a given length representing the coupling region and approximately equal to an odd multiple of a quarter wavelength of the operating microwave signal frequency of the associated communication system. Similarly, the directional coupler 3 has two longitudinally-extending copper strips 12 and 13, identical with the strips 10 and 11 of coupler 3, embedded in the lower surface (bottom ground plane) of the dielectric sheet 9, including two straight central portions extending longitudinally in closely parallel relation to each other for a given length representing the coupling region and approximately equal to an odd multiple of a quarter wavelength of the operating microwave frequency of the associated communication system. The dimensions of, and spacing between, the coupled copper strips or strip lines in each of the directional couplers 3 and 4 are such that each of these couplers has a minimum coupling of -3 decibels over a 2:1 frequency band. The frequency range of each of these couplers may be 8500 to 12,000 mc., for example.

The transmitter 5 of the radar or like radio communication system is connected through the 50-ohm coaxial connector 14 to one terminal 15 of the strip line 13 of directional coupler 3, and the common transmitting and receiving antenna 7 of that system is connected through a similar coaxial connector 16 to the adjacent terminal 17 of the strip line 12 of directional coupler 3. The receiver 6 of the communication system is connected through the similar coaxial connector 18 to one terminal 19 of the strip line 10 of the directional coupler 4, and a non-reflective dummy load 8 made from any suitable resistance material and of a resistance value sufiicient to match the impedance of the antenna 7 is connected to the adjacent terminal 22 of the strip line 11 of the directional coupler 4. The active element, the crystal garnet sphere or bead 1a, of the resonator 1 is connected directly to the other terminal 23 of the strip line 13 of the directional coupler 3, and one end of the wire conductor 1b of that resonator, coupled to the bead 1a, is connected directly to the terminal 24 of the strip line 11 of directional coupler 4, so as to provide a conductive loop connection through the resonator 1 between these terminals of the two directional couplers 3 and 4. Similarly, the active element, crystal garnet sphere or bead 2a, of the resonator 2 is connected directly to the other terminal 25 of the strip line 12 of the directional coupler 3, and one end of the wire conductor 2b of that resonator is connected directly to the other terminal 26 of the strip line 10 of directional coupler 4, so as to provide a conductive loop connection through the resonator 2 between the latter terminals of the two directional couplers 3 and 4.

The balanced duplexer of the invention as shown in FIG. 2 and described above operates as follows: The high-power outgoing signal energy of microwave frequency generated by the transmitter 5 of the associated radar or radio communication system in each signal transmitting interval will be supplied through the coaxial connector 14 to the terminal 15 of the directional coupler 3 which, because of the coupling between its two strip lines 12 and 13, will operate to split the supplied energy into two equal energy portions differing in phase by electrical degrees. These two outgoing signal energy portions will emerge at the terminals 23 and 25, respectively, of that coupler and will be respectively applied to the conductive loop formed by the coupling of the head In and wire conductor 1b of resonator 1, and to the conductive loop formed by the coupling of the bead 2a and the wire conductor 2b of resonator 2. The effect of the incident high-power signal outgoing signal energy portions on the garnet material in the active elements 1a and 2a resonators will be to increase the impedance of these elements to a relatively high value and thus provide a power-limiting action on these signal energy portions such that substantially none of the high-power signal energy will be transmitted through the resonators and the associated strip lines 10 and 11 to the receiver 6 and dummy load 8. Thus, the sensitive detector elements in receiver 6 will be protected from the high-power outgoing signal energy. Instead, the high impedance condition of the garnet beads 1a and 2a will cause reflection of substantially all of the energy of the incident high-power outgoing signal energy portions and be reflected back through terminals 23 and 25, respectively, into the copper strip lines 12 and 13 of directional coupler 3. The reflected outgoing signal energy portions will be combined in phase and amplitude by coupler 3 in such manner that two reflected energy portions appearing at the terminal 22 of the coupler connected to the dummy load 8 will be equal in amplitude and opposite in phase and will cancel out, whereas the two reflected energy portions of the outgoing signal energy appearing at terminal 17 0f coupler 3 connected to the antenna 7 will be of equal amplitude and in the same phase so as to reinforce each other. Thus, substantially all of the reflected outgoing signal energy will be transmitted to antenna 7 and will be radiated thereby.

The action of the balanced duplexer of the invention in the signal transmitting condition is illustrated by the curve of FIG. 3 showing the signal power out of the transmitter plotted against the signal power produced in the receiver arm of the duplexer bridge. As shown on this figure, the power appearing in the receiver arm in response to the outgoing signal power of the transmitter will never exceed the safe limiting value of the detector elements in the receiver, whereas the signal power entering the antenna arm of the bridge will be essentially equal to the power out of the transmitter.

During the alternate signal receiving intervals of the associated radar or similar radio system, the relatively low-power incoming echo or radio pulse energy, of the same microwave frequency as the operating frequency of the transmitter, picked up by the antenna 7 of the system will be applied through coaxial connector 16 to terminal 17 of directional coupler 3 and, due to the coupling provided between the copper strip lines 12 and 13 of that coupler, will be split into two equal energy portions differing in phase by 90 electrical degrees which will respectively emerge at terminals 23 and 25 of the coupler 3 and will be applied to the conductive loops formed by the coupling of the ferrite (crystal garnet) active element la to wire conductor 1b of resonator 1 and the identical active element 2a to the Wire conductor 2b of resonator 2, respectively. As the power level of the incident incoming signal energy portions is relatively low and the resonators 1 and 2 are tuned to their frequency by the applied direct current magnetic field, the impedances of the active elements of the resonators 1 and 2 will be maintained at their normal relatively low value for this condition. The filter actions of the resonators 1 and 22 will predominate with the result that the incident incoming signal energy portions will be transmitted through these resonators operating as filters with low loss only in the narrow frequency range to which the resonators are tuned. The outputs of the two resonators 1 and 2 for this condition will be applied through the terminals 24 and 26, respectively, to the copper strip lines and 11 of directional coupler 4. The coupler 4 will operate the split each of the supplied incoming signal energy portions into two equal portions one of which will emerge at terminal 22 of that coupler connected to the dummy load 8 and the other at terminal 19 thereof connected through coaxial connector 18 to the receiver 6. The two equal incoming signal energy portions emerging at terminal 22 of coupler 4 will be opposite in phase and will cancel. Any signal due to the departure from ideal coupling will be dissipated in the associated dummy load 8, whereas the two equal incoming signal energy portions emerging lat terminal 19 of coupler 4 will be in phase and will therefore reinforce each other. The resulting combined incoming signal energy will pass with low loss to the receiver 6 and will be detected therein.

The desired frequency range of the resonators 1 and 2 operating as narrow-band filters in the low incident power condition can be tuned over a wide microwave frequency band depending on the adjustment of the strength of the applied direct current magnetic field, as illustrated by the curve of FIG. 4 showing the attenuation produced in the receiver arm of the balanced bridge when the resonators are tuned to different frequencies f f in this wide range for different strengths H H;,,- of the applied D.C. magnetic field.

Although, in the duplexer of the invention as described and illustrated, the active elements of the resonators employ single crystal garnet material in the form of small spheres or beads, it is to be understood that the invention is not to be limited to this particular material and shape of the elements and that any other fem'te or other ferrimagnetic material in other shapes which would provide the same filter action and the same filter-limiting action in response to incident low-power and high-power signal energy, respectively, could be used in accordance with the invention. Also, directional couplers of other known types, for example, of the conventional short-slot hybrid junction type, could be employed in the balanced duplexer of the invention in place of the directional couplers of the coupled-strip-line type which have been illustrated and described. Other modifications of the circuits illustrated and described which are within the spirit and scope of the invention will occur to persons skilled in the art.

What is claimed is:

In combination with a radio communication system including a transmitter and a receiver tor respectively generating high-power outgoing signals of microwave frequency and detecting relatively low-power incoming signals of the same frequency, a common transmitting and receiving antenna and duplexing means connecting said antenna to said transmitter and receiver for conditioning said system alterately tor signal transmission and reception, said duplexing means comprising two branching circuits each including a single crystal garnet resonator in the form of a spherical bead and positioned in gyromagnetic coupling relationship between two orthogonal transmission lines, the impedance of said single crystal garnet resonators in response to the high-power outgoing signals incident thereon in signal transmitting intervals being high and causing said resonators to provide a power-limiting action on these signals such as to prevent transmission therethrough of any appreciable amount of their energy and instead to cause that energy to be reflected by the resonators, the impedance of said single crystal garnet resonators in signal receiving intervals when the relatively low-power incoming signals are incident thereon being low causing said resonators to operate as a narrow-band filter tuned to said microwave frequency to transmit therethrough substantially all of the incoming signal energy in its frequently-band range with low loss, and directional coupler means connected to said orthogonal transmission lines and operating to direct substantially all of the reflected outgoing signal energy in signal transmitting intervals to said antenna for radiation thereby and substantially all of the filtered incoming signal energy through said resonators in signal receiving intervals to said receiver tor detection therein.

References Cited in the file of this patent UNITED STATES PATENTS 2,586,993 Riblet Feb. 26, 1952 2,849,685 Weiss Aug. 26, 1958 2,850,624 Kales r Sept. 2, 1958 2,916,712 Artuso Dec. 8, 1959 2,920,292 Scovil et al. Jan. 5, 1960 

